The analysis of oxygen, saccharide, amino acid, pH etc. has hitherto been done by an electrochemical method, using several kinds of electrodes. Following is the explanation of prior techniques for various kinds of electrodes and sensors.
(1) Sensor for measuring the concentration of dissolved oxygen
It is important to measure the concentration of dissolved oxygen in a body fluid such as blood, or that of a culture fluid in a fermenter jar. Clark type oxygen sensor has been used widely. The composition of this sensor is as follows.
The container of the sensor is filled with an electrolyte such as a caustic alkaline solution etc., and the cathode (for example made of platinum) and the anode (for example made of lead etc.) are set therein, and separated from outside (the solution which should be measured) by a gas permeable membrane made of polytetrafluoroethylene etc.. The concentration of oxygen can be determined by measuring the electric current between the two electrodes, whereas oxygen molecules pass from the test solution to the inside of the sensor container through a selective permeable membrane, and is reduced electrochemically on the platinum electrode surface. By the appearance of this method, the amount of oxygen can be determined easily, but there are still the following problems such as, instability of pressure at sterilization, difficulty of maintenance, limitation for miniaturization, inability to determine the real dissolved oxygen concentration, except for partial oxygen pressure, inability to determine the dissolved oxygen concentration in fermentation under pressure or on the bottom of the sea and difficulty because the lakes, and the variation of the electric current is rather large in relation to temperature change.
Several methods are proposed for other oxygen concentration measuring electrodes, especially for body fluids. One is a method to make the condition of stable contact by means of covering the fine metal wire electrode surface with porous materials consisting of many layers (see Japanese Unexamined Patent Publication No. 57-117838). Another is a method to insert the fine metal wire electrode into the opened tube in the recessed position from the tip of the said electrode (see Japanese Unexamined Patent Publication No. 57-195436).
However, these methods have still several problems such as contamination of the electrode and the possibility of suffering from iatrogenic disease by peeling off the porous membrane.
(2) The sensor for measuring the concentration of ions A glass electrode has been widely used for measuring the concentration of hydrogen ions. However, this has the following several problems. That is, it is easy to break, also it is easy to contaminate, and it has limitations when used in alkali aqueous solution. Moreover, it is very difficult to miniaturize, because the glass electrode needs an inside standard solution compartment. And there is a possibility that the inside standard solution may flow out.
Recently, the following phenomena has been discovered: A membrane, which is deposited by means of electrochemical polymerization on the surface of the electrode, causes a change in potential in accordance with bonding or removing the hydrogen ion. [William R. Heineman, Anal. Chem. 52, 345 (1980)] The following pH sensor was also proposed. The method has applied the following phenomena wherein the membrane, which is formed on the surface of an electrode, causes a reversible oxidation/reduction reaction with bonding or removing of hydrogen ion (see Japanese Unexamined Patent Publication No. 61-19434). However, when these pH sensors are used continuously in body fluids or fermentative solution, a potential drift occurs in accordance with contamination.
(3) Bio-sensor
There are two types of bio-sensors. That is, the first one is used to determine the concentration of materials related to a biochemical reaction, by measuring the current caused in an electrochemical reaction of said materials.
The other one is to analyze the electroactive materials (related to biochemical reaction), by measuring the potential of the membrane which selectively responds to said materials. The former one is called an "Amperometry method" and in this case, an oxygen sensor and a hydrogen peroxide electrode are applied. On the other hand, the latter one is called a "Potentiometry type" and an ion selective electrode and an ammonia or carbon dioxide gas electrode (which consists of some kind of gas permeable membrane and pH electrode) are applied.
Enzyme, microorganism, and antigenantibody reaction are one of the biochemical reactions. The bio-sensor has problems as said hereinbefore, whereas a fundamental composition of a bio-sensor is an oxygen sensor or a pH electrode.
(4) The reference electrode
Conventional reference electrodes have the following problems; 1) they are easy to break, 2) they are hard to ensure for the pressure of sterilization, whereas the saturated calomel electrode and silver/silver chloride electrode etc. includes an inside-solution.
Therefore, an object of the present invention is to provide various types of electrodes and also sensors which can determine for example, the dissolved oxygen, saccharide, amino acid, pH etc. stably and accurately without disadvantages of prior techniques.